Optical fiber

ABSTRACT

The present invention relates to an optical fiber having, at least, a structure for effectively restraining microbend loss from increasing. This optical fiber is an optical fiber suitable for a dispersion-flattened fiber, a dispersion-compensating fiber, and the like, and insured its single mode in a wavelength band in use. In particular, since the fiber diameter is 140 μm or more, this optical fiber has a high rigidity, so that the increase in microbend loss is effectively suppressed, whereas the probability of the optical fiber breaking due to bending stresses is practically unproblematic since the fiber diameter is 200 μm or less. Also, since this optical fiber has a larger mode field diameter, it lowers the optical energy density per unit cross-sectional area, thereby effectively restraining nonlinear optical phenomena from occurring.

RELATED APPLICATIONS

This is a Continuation-In-Part application of application Ser. No.PCT/JP99/03672 filed on Jul. 7, 1999, now WO00/02074 pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical fiber which is suitable asan optical transmission line in wavelength division multiplexing (WDM)transmission systems.

2. Related Background Art

WDM transmission lines which enable optical transmissions, those of alarge capacity and high speed in particular, mainly utilize opticalfibers. Recently, however, the deterioration in light signals caused bynonlinear optical phenomena such as four-wave mixing among individuallight signals occurring in an optical fiber has become problematic insuch WDM transmission systems. Therefore, in the WDM transmissionsystems, it is important that the occurrence of nonlinear opticalphenomena be suppressed, and to this aim, it is necessary that the modefield diameter or effective area of the optical fiber be increased, soas to lower the optical energy density per unit cross-sectional area.For example, Japanese Patent Application Laid-Open No. HEI 8-248251discloses an optical fiber having an effective area (70 μm² or more)which is greater than that of normal dispersion-shifted fibers.

SUMMARY OF THE INVENTION

It has been known that, in general, microbend characteristicsdeteriorate as the mode field diameter or the effective area increases,whereby the microbend loss caused by cabling becomes greater.

For example, FIG. 1 is a chart showing the refractive index profile of adispersion-shifted fiber having a double core structure. In thisdispersion-shifted fiber, the core region is constituted by an innercore having a refractive index n1 and an outer core having a refractiveindex n2 (<n1), whereas a single cladding layer having a refractiveindex n3 (<n2) is provided on the outer periphery of the core region. Onthe other hand, FIG. 2 is a graph showing the relationship between themode field diameter and the increase in loss caused by microbend at awavelength of 1.55 μm (1550 nm) concerning this optical fiber having therefractive index profile of a double core structure. In thisspecification, the mode field diameter refers to Petermann-I mode fielddiameter. This Petermann-I mode field diameter is given by the followingexpressions (1a) and (1b):

MFD1=2·w₁  (1a)

$\begin{matrix}{w_{1}^{2} = {2 \cdot \frac{\int_{0}^{\infty}{{E^{2} \cdot r^{3}}{r}}}{\int_{0}^{\infty}{{E \cdot r}{r}}}}} & \text{(1b)}\end{matrix}$

as shown in E. G. Neumann, “Single-Mode Fibers,” pp. 225, 1988.

In expression (1b), r is the radial positional variable from the corecenter taken as the origin, whereas E is the electric field amplitudeand is a function of the positional variable r. The microbend loss isdefined by the amount of increase in loss when an optical fiber having alength of 250 m is wound at a tension of 100 g around a bobbin having abarrel diameter of 280 mm whose surface is wrapped with a JIS #1000sandpaper sheet.

Also, from the results of theoretical studies, it has been known thatthe relationships of the following expressions (2a) to (2c) existbetween microbend loss Δ α and mode field diameter MFD1: $\begin{matrix}{{\Delta\alpha} = {\frac{1}{4} \cdot \left( \frac{1}{R^{2}} \right) \cdot \left( {k \cdot {n1} \cdot w_{1}} \right)^{2} \cdot {\Phi ({\Delta\beta})}}} & \text{(2a)} \\{{\Delta\beta} = \frac{1}{w_{1}^{2} \cdot k \cdot n_{1}}} & \text{(2b)} \\{{\Phi ({\Delta\beta})} = {\pi^{1/2} \cdot {Lc} \cdot {\exp \left\lbrack {- \left( \frac{{\Delta\beta} \cdot {Lc}}{2} \right)^{2}} \right\rbrack}}} & \text{(2c)}\end{matrix}$

In these expressions, R is the radius of curvature of microbendingapplied to the optical fiber, k is the wave number, n1 is the refractiveindex of the core region, and Lc is the correlation length of themicrobending applied to the optical fiber.

As can be seen from FIG. 2 and expressions (2a) to (2c) mentioned above,the microbend loss increases as the mode field diameter MFD1 is greater.However, though the conventional optical fibers are designed in view ofmacrobend loss, no consideration has been given to microbend loss. Also,it has been known that, if the amount of increase in loss measured whenan optical fiber is wound around a bobbin whose surface is wrapped withsandpaper, as an index for cabling an optical fiber, exceeds about 1dB/km, then microbend loss increases upon cabling. Hence, it is clearthat microbend loss increases upon cabling in an optical fiber such asthe one mentioned above.

In order to overcome such problems, it is an object of the presentinvention to provide an optical fiber having, at least, a structurewhich can effectively suppress the increase in microbend loss.

For achieving the above-mentioned object, the optical fiber according tothe present invention comprises a core region extending along apredetermined axis and a cladding region provided on the outer peripheryof the core region, these core and cladding regions being constituted byat least three layers of glass regions having respective refractiveindices different from each other. Also, this optical fiber issubstantially insured its single mode with respect to light at awavelength in use, e.g., in a 1.55-μm wavelength band (1500 nm to 1600nm), and has a fiber diameter of 140 μm or more about 200 μm or less.Thus, since the fiber diameter is 140 μm or more, the rigidity of theoptical fiber according to the present invention is high even when themode field diameter is large, whereby the increase in microbend loss issuppressed. On the other hand, since the fiber diameter is not greaterthan 200 μm, the probability of the optical fiber breaking due tobending stresses is practically unproblematic.

In particular, when the 1.55-μm wavelength band is employed as thewavelength band in use for WDM transmissions, it is preferred in theoptical fiber according to the present invention that the absolute valueof chromatic dispersion at a wavelength of 1550 nm be 5 ps/nm/km orless. Also, it is preferred that the Petermann-I mode field diameter be11 μm or more. It is because of the fact that, if the mode fielddiameter is 11 μm or more, then the optical energy density per unitcross-sectional becomes smaller even when WDM signals are transmitted,whereby the occurrence of nonlinear optical phenomena can effectively besuppressed.

The optical fiber according to the present invention can be employed asa single-mode optical fiber such as dispersion-shifted fiber,dispersion-flattened fiber, dispersion-compensating fiber, or the like.

In particular, when the optical fiber according to the present inventionis employed as a dispersion-flattened fiber, it is preferable for theoptical fiber to have, for at least one wavelength within the wavelengthband in use, a dispersion slope of 0.02 ps/nm²/km or less and aneffective area of 50 μm² or more. More preferably, in particular, thedispersion slope is 0.02 ps/nm²/km or less in terms of absolute value.

Also, when the optical fiber according to the present invention isemployed as a dispersion-compensating fiber, it is preferable for theoptical fiber to have, for at least one wavelength within the wavelengthband in use, a chromatic dispersion of −18 ps/nm/km or less and aneffective area of 17 μm² or more.

Further, when the optical fiber according to the present invention isemployed as an optical fiber having an enlarged effective area, it ispreferable for the optical fiber to have, for at least one wavelengthwithin the wavelength band in use, an effective area of 110 μm² or more.The optical energy density per unit cross-sectional area can be kept lowin this optical fiber as well, whereby the occurrence of nonlinearoptical phenomena can be suppressed effectively.

In various kinds of optical fibers mentioned above, the fiber diameteris 150 μm or more but 200 μm or less. In the case of adispersion-compensating fiber having such characteristics as thosementioned above, however, its fiber diameter is preferably 140 μm ormore but 200 μm or less since its microbend characteristics are likelyto deteriorate in particular.

When the optical fiber according to the present invention is employed inan optical cable, it is preferable for the optical fiber to have, for atleast one wavelength within the wavelength band in use, an effectivearea of 17 μm² or more and a chromatic dispersion value of −83 ps/nm/kmor more, and have a fiber diameter of 140 μm or more but 200 μm or less.Such an optical fiber aimed at cabling can be employed as a single-modeoptical fiber such as dispersion-shifted fiber, dispersion-flattenedfiber, dispersion-compensating fiber, or the like as well.

As explained in the foregoing, in view of various circumstancesapplicable thereto, the optical fiber according to the present inventionis preferably an optical fiber which has a fiber diameter of 140 μm ormore but 200 μm or less, and also has, for at least one wavelengthwithin the wavelength band in use, an effective area of 17 μm² or moreand a chromatic dispersion value of −83 ps/nm/km or more; and, further,preferably is an optical fiber which has a fiber diameter of 140 μm ormore but 200 μm or less, and also has, for at least one wavelengthwithin the wavelength band in use, an effective area of 17 μm² or moreand a chromatic dispersion value of −48 ps/nm/km or more. Also,depending on the kind of optical fiber employed, the fiber diameterthereof is preferably 150 μm or more but 200 μm or less.

The present invention will be more fully understood from the detaileddescription given hereinbelow and the accompanying drawings, which aregiven byway of illustration only and are not to be considered aslimiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will beapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart showing the refractive index profile (double corestructure) of a dispersion-shifted fiber;

FIG. 2 is a graph showing the relationship between the mode fielddiameter (Petermann-I) and the increase in loss caused by microbend at awavelength of 1.55 μm in the dispersion-shifted fiber shown in FIG. 1;

FIG. 3A is a view showing a cross-sectional structure which is common inindividual embodiments of the optical fiber according to the presentinvention, whereas FIG. 3B is a chart showing the refractive indexprofile of the optical fiber according to the fourth embodiment;

FIG. 4 is a table listing characteristics of four samples prepared asprototypes for explaining the optical fiber according to the firstembodiment;

FIG. 5 is a graph showing respective results of evaluation of the foursamples prepared as prototypes for explaining the optical fiberaccording to the first embodiment;

FIG. 6A is a view showing a cross-sectional structure of an opticalfiber unit constituting a part of an optical cable, whereas FIG. 6B is aview showing a cross-sectional structure of the optical cable having theoptical fiber unit shown in FIG. 6A;

FIG. 7 is a table listing characteristics of three samples prepared asprototypes for explaining the second embodiment of the optical fiberaccording to the present invention;

FIG. 8 is a table listing characteristics of four samples prepared asprototypes for explaining the third embodiment of the optical fiberaccording to the present invention;

FIG. 9 is a graph showing respective results of evaluation of the foursamples prepared as prototypes for explaining the optical fiberaccording to the third embodiment;

FIG. 10 is a graph showing a relationship between fiber diameter andprobability of breaking;

FIG. 11 is a table listing characteristics of two samples prepared asprototypes for explaining the fourth embodiment of the optical fiberaccording to the present invention;

FIG. 12 is a chart showing a refractive index profile in the fifthembodiment of the optical fiber according to the present invention;

FIG. 13 is a chart showing a refractive index profile in the sixth andseventh embodiments of the optical fiber according to the presentinvention; and

FIG. 14 is a graph showing the relationship between chromatic dispersionand dispersion slope in the optical fiber (Δ⁺=0.9%, Δ⁻=−0.44% )according to the sixth embodiment shown in FIG. 13.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Individual embodiments of the optical fiber according to the presentinvention will now be explained in detail with reference to FIGS. 1, 3A,3B, 4, 5, 6A, 6B, and 7 to 14.

Here, as shown in FIG. 3A, the optical fiber 100 according to thepresent invention comprises a core region 110 extending along apredetermined axis and having an outside diameter a, and a claddingregion 120 provided on the outer periphery of the core region 110 andhaving an outside diameter b (coinciding with the fiber diameter),whereas the core and cladding regions are constituted by at least threelayers of glass regions having respective refractive indices differentfrom each other in embodiments which will be explained in the following.Also, in each of the optical fibers according to the respectiveembodiments, the absolute value of chromatic dispersion at a wavelengthof 1.55 μm (1550 nm) is 5 ps/nm/km or less, the Petermann-I mode fielddiameter is 11 μm or more, and the fiber diameter b is 140 μm or morebut 200 μm or less.

The optical fibers according to the first to third embodiments have arefractive index profile of double core structure identical to thatshown in FIG. 1, whereas the optical fiber according to the fourthembodiment has a refractive index profile with an outer ringcore/depressed cladding structure as shown in FIG. 3B.

The refractive index profile shown in FIG. 1 indicates the refractiveindex in each part on the line L in FIG. 3A. In the optical fibers ofthe first to third embodiments, the core region 110 having the outsidediameter a is constituted by an inner core having a refractive index n1,and an outside core provided on the outer periphery of the inner coreand having a refractive index n2 (<n1), whereas the cladding region 120having the outside diameter b (coinciding with the fiber diameter) isconstituted by a single cladding provided on the outer periphery of theouter core and having a refractive index n3 (<n2). Thus, each of theoptical fibers according to the first to third embodiments is an opticalfiber which is constituted by three glass layers (the inner core, outercore, and single cladding) and is insured its single mode in awavelength band in use.

On the other hand, the optical fiber according to the fourth embodimentis an optical fiber having a refractive index profile 500 with an outerring core/depressed cladding structure as shown in FIG. 3B, whereas therefractive index profile 500 also indicates the refractive index in eachpart on the line L in FIG. 3A. In particular, in the refractive indexprofile 500, parts 510 and 520 indicate a core region having an outsidediameter a and a cladding region having an outside diameter b,respectively. In the fourth embodiment, the core region is constitutedby an inner core having a refractive index n1, an intermediate coreprovided on the outer periphery of the inner core and having arefractive index n2 (<n1), and an outer ring core provided on the outerperiphery of the intermediate core and having a refractive index n3(>n2). On the other hand, the cladding region is constituted by an innercladding provided on the outer periphery of the outer core and having arefractive index n4 (<n3), and an outer cladding provided on the outerperiphery of the inner cladding and having a refractive index n5 (>n4).Thus, the optical fiber according to the fourth embodiment is an opticalfiber which is constituted by five layers of glass (the inner core,intermediate core, outer ring core, inner cladding, and outer cladding)and is insured its single mode in a wavelength band in use.

The optical fibers according to the first to seventh embodiments havingthe refractive index profiles mentioned above will now be explainedsuccessively.

FIRST EMBODIMENT

First, for explaining the optical fiber according to the firstembodiment, four kinds of optical fibers (sample 1a to sample 1d) havingsubstantially the same Petermann-I mode field diameter MFD1 andrespective values of fiber diameter b different from each other wereprepared as prototypes and evaluated. FIG. 4 is a table listingcharacteristics of each of the four kinds of samples 1a to 1d preparedas prototypes for explaining the optical fiber according to the firstembodiment.

The fiber diameter b is about 125 μm in sample 1a, about 140 μm insample 1b, about 150 μm in sample 1c, and about 160 μm in sample 1d.Here, each of the Petermann-I mode field diameter MFD1 (11.73 to 11.88μm) given by the above-mentioned expressions (1a) and (1b), effectivearea (69.7 to 72.1 μm²),chromatic dispersion value (−2.2 to −1.9ps/nm/km), and cutoff wavelength (1.50 to 1.53 μm) is substantially thesame among the four kinds of samples la to 1d.

Such four kinds of samples 1a to 1d having respective values of fiberdiameter b different from each other are obtained by preparing fourkinds of preforms, which use core members having an identical diameter,whose outside diameter ratios between core member and cladding memberdiffer from each other, and drawing them. Further, the periphery of eachof the four kinds of samples 1a to 1d is provided with a coating layer,made of the same material, having an outside diameter of 250 μm. Thevalues of the mode field diameter MFD1, effective area, and chromaticdispersion are those measured at a wavelength of 1.55 μm (1550 nm).

For each of these four kinds of samples 1a to 1d, microbend loss wasmeasured. In this measurement, each optical fiber having a length of 250m was wound at a tension of 100 g around a bobbin having a barreldiameter of 280 mm whose surface was wrapped with a JIS #1000 sandpapersheet, and the resulting amount of increase in loss was defined as themicrobend loss. FIG. 5 is a graph showing the respective results ofevaluation of the four kinds of samples prepared as prototypes forexplaining the optical fiber according to the first embodiment. It isseen from this graph that the amount of increase in loss, i.e.,microbend loss, becomes smaller as the fiber diameter b is greater.

When an optical cable having a cross-sectional structure such as thoseshown in FIGS. 6A and 6B is being made, the microbend loss increasesupon cabling. For preventing this phenomenon from occurring, it isnecessary that the microbend loss be suppressed to about 1 dB/km orless. Therefore, a scan be seen from FIG. 5, it is necessary for thefiber diameter b to become 140 μm or more when the effective area isabout 70 μm².

Here, FIG. 6A is a view showing the cross-sectional structure of anoptical fiber unit, whereas FIG. 6B is a view showing thecross-sectional structure of an optical cable in which this opticalfiber unit 200 is employed. In FIG. 6A, optical fibers 100 each having aUV-curable resin 150 coated thereon as a coating layer are integratedwith each other about a tension member 151 with a UV-curable resin 152.Further, the periphery of the UV-curable resin 152 is coated with aUV-curable resin 153, so that the optical fiber unit 200 is obtained. Anoptical cable 300 employing the optical fiber unit 200 is obtained byaccommodating into a copper tube 253 the optical fiber unit 200successively covered with a three-part pipe 250 made of a steel andtension piano wires 252 by way of a water-running prevention compound251, and then successively covering the outer periphery of the coppertube 253 with a low-density polyethylene 254 and a high-densitypolyethylene 255.

SECOND EMBODIMENT

Next, in the second embodiment, the comparison between characteristicsof a conventional optical fiber which has already been verified to befree from the problem caused by cabling and characteristics of theoptical fiber according to the second embodiment will be explained. FIG.7 is a table listing characteristics of each of three kinds of samples2a to 2c prepared as prototypes for explaining the optical fiberaccording to the second embodiment. Among them, sample 2a is aconventional optical fiber which has already been verified to be freefrom the problem caused by cabling.

The fiber diameter b is about 125 μm in samples 2a and 2b, and about 140μm in sample 2c. The Petermann-I mode field diameter MFD1 given by theabove-mentioned expressions (1a) and (1b) is about 10 μm in sample 2a,and about 11 μm in samples 2b and 2c. The effective area is about 55 μm² in sample 2a, and about 65 μm² in samples 2b and 2c. Here, each of thechromatic dispersion value (−2.2 to −2.0 ps/nm/km) and cutoff wavelength(1.51 to 1.53 μm) is substantially the same value among the three kindsof samples 2a to 2c. The periphery of each of the three samples 2a to 2cis provided with a coating layer, made of the same material, having anoutside diameter of 250 μm. The values of the mode field diameter MFD1,effective area, and chromatic dispersion are those measured at awavelength of 1.55 μm.

Using these three kinds of samples 2a to 2c, the optical fiber unit 200having the cross-sectional structure shown in FIGS. 6A and 6B was made.A hydraulic pressure of 100 atmospheres was applied to thus obtainedoptical fiber unit 200, so as to simulate the external pressure appliedto the optical fiber unit 200 upon cabling. As a result, the amount ofincrease in loss, i.e., microbend loss, in sample 2b having a fiberdiameter b of about 125 μm and an effective area of about 65 μm² was 20mdB/km. On the other hand, the amount of increase in loss, i.e.,microbend loss, in sample 2c having a fiber diameter b of about 140 μmand an effective area of about 65 μm² was not greater than 0.5 mdB/km,which was the limit of measurement, whereby this sample yielded acharacteristic equivalent to that of the conventional sample 2a whichhad already been verified to be free from the problem caused by cabling.

THIRD EMBODIMENT

The third embodiment will now be explained. In the third embodiment, foroptical fibers having a further greater effective area, the effect ofreducing microbend loss resulting from the increase in fiber diameterwas verified. FIG. 8 is a table listing characteristics of each of fourkinds of samples 3a to 3d prepared as prototypes for explaining theoptical fiber according to the third embodiment.

The fiber diameter b is about 150 μm in samples 3a and 3c, and about 170μm in samples 3b and 3d. The Petermann-I mode field diameter MFD1 givenby the above-mentioned expressions (1a) and (1b) is about 13.2 μm insamples 3a and 3b, and about 14.2 μm in samples 3c and 3d. The effectivearea is about 80 μm² in samples 3a and 3b, and about 90 μm ² in samples3c and 3d. On the other hand, each of the chromatic dispersion value(−2.0 to −2.2 ps/nm/km) and cutoff wavelength (1.50 to 1.53 μm) issubstantially the same value among the four kinds of samples 3a to 3d.The periphery of each of the four samples 3a to 3d is provided with acoating layer, made of the same material, having an outside diameter of250 μm. The values of the mode field diameter MFD1, effective area, andchromatic dispersion are those measured at a wavelength of 1.55 μm.

For each of these four kinds of samples 3a to 3d, microbend loss wasmeasured. The method of measuring the microbend loss was similar to thatin the case of the first embodiment. FIG. 9 is a graph showingrespective results of evaluation of the four kinds of samples preparedas prototypes for explaining the optical fiber according to the thirdembodiment. As can be seen from this graph, when the effective area isabout 80 μm², the microbend loss can be suppressed to its target valueof about 1 dB/km or less if the fiber diameter b is about 150 μm ormore. On the other hand, when the effective area is about 90 μm², themicrobend loss can be suppressed to its target value of 1 dB/km or lessif the fiber diameter b is about 170 μm or more.

From the foregoing, it is seen that, even when the effective area islarge, the microbend loss can be suppressed to its target value of about1 dB/km or less if the fiber diameter is large. This fact can beexplained by the following as well. Namely, the microbend loss of anoptical fiber is generated when an external force applied to the opticalfiber causes random minute curvatures in the longitudinal direction inthe core region of the optical fiber. This microbend loss isproportional to the square mean of the reciprocals of the radii ofcurvature of the minute curvatures. On the other hand, if the externalforce applied to the optical fiber is constant, then the minutecurvatures occurring in the optical fiber can be suppressed byincreasing the rigidity of the optical fiber. Letting b be the fiberdiameter of the optical fiber, the rigidity (bending moment) I of theoptical fiber is given by the following expression (3):

I=π·b⁴/64  (3)

Therefore, increasing the fiber diameter b of the optical fiber greatlyimproves the rigidity I of the optical fiber, thereby suppressing minutecurvatures and greatly reducing the microbend loss.

For example, when an optical fiber having a fiber diameter of 200.3 μm,an effective area of 90.4 μm², a chromatic dispersion value of −2.1ps/nm/km, a cutoff wavelength of 1.73 μm, and a coating layer outsidediameter of 250 μm was prepared as a prototype, and its microbend losswas measured in a manner similar to that of the first embodiment, theresulting value was 0.3 dB/km. This characteristic is on a par with thatof the conventional optical fiber which has already been verified to befree from the problem caused by cabling.

As the fiber diameter b increases, however, the stress in the fibersurface (the surface of the outermost cladding layer) occurring when theoptical fiber is bent becomes greater than that in the conventionaloptical fiber, thereby increasing the probability of breaking uponbending. Hence, the range of fiber diameter b within which theprobability of breaking is practically unproblematic will now becalculated by way of trial. The probability of breaking F of the opticalfiber after passing a screening test is given by the followingexpression (4): $\begin{matrix}{F = {1 - {\exp \left( {{- L} \cdot {Np} \cdot \left\{ {\left( \left\lbrack {1 + {\left( \frac{\sigma_{s}}{\sigma_{p}} \right)^{n} \cdot \frac{ts}{tp}}} \right\rbrack \right)^{m/{({n + 1})}} - 1} \right\}} \right)}}} & (4)\end{matrix}$

where L is the optical fiber length to which a stress is applied in thestate of its actual use, Np is the number of breaks per unit length atthe time of screening test, σs is the stress upon its actual use, σp isthe stress at the time of screening, ts is the time of actual use, tp isthe screening time, n is the fatigue coefficient, and m is a parameterrepresenting the gradient of a Weibull plot.

It is assumed that smaller-diameter bending in an optical fiber at thetime of actual use occurs in a surplus length portion which is left forfusion connection in a repeater, and that one turn of bending with adiameter of 30 mm exists in one repeater at worst. Also, it is assumedthat the total optical fiber length of the optical transmission systemis 9000 km, in which repeaters are disposed at intervals of 50 km onaverage. Then, the fiber length L to which the bending with a diameterof 30 mm is applied becomes 16.9 m in the optical transmission system asa whole. It is also assumed that the number of breaks per unit length atthe time of screening test Np is 2×10⁻⁵, and the stress σp at the timeof screening is 2.2%. Let the time of actual use ts be 25 years, and thescreening time tp be 1 second. Let the fatigue coefficient n be 20, andthe parameter m representing the gradient of the Weibull plot be 10.

FIG. 10 is a graph showing the relationship between the fiber diameter band the probability of breaking according to above-mentioned expression(4) on the foregoing assumption. As can be seen from this graph, theprobability of breaking becomes higher as the fiber diameter is greater.If the fiber diameter is 200 μm or less, however, then the probabilityof breaking is 10⁻⁵ or lower, whereby there is no problem in practice.

FOURTH EMBODIMENT

The fourth embodiment will now be explained. Prepared for the evaluationof the fourth embodiment were two kinds of samples 4a and 4b, eachhaving a refractive index profile of a dispersion-shifted optical fiberwith the outer ring core/depressed cladding structure shown in FIG. 3B,whose values of Petermann-I mode field diameter MFD1 were substantiallyidentical to each other, whereas their values of fiber diameter b weredifferent from each other.

Here, as mentioned above, the refractive index n1 of the inner core, therefractive index n2 of the intermediate core, the refractive index n3 ofthe outer ring core, the refractive index n4 of the inner cladding, andthe refractive index n5 of the outer cladding have relationships ofn1>n2, n3>n2, and n5>n4.

FIG. 11 is a table listing characteristics of each of the two kinds ofsamples prepared as prototypes for explaining the optical fiberaccording to the fourth embodiment. The fiber diameter b is about 125 μmin sample 4a, and about 150 μm in sample 4b. On the other hand, each ofthe Petermann-I mode field diameter MFD1 (11.98 and 12.17 μm) given bythe above-mentioned expressions (1a) and (1b), effective area (69.7 and72.1 μm²), chromatic dispersion value (−2.1 and −2.2 ps/nm/km), andcutoff wavelength (1.53 and 1.51 μm) is substantially the same valuebetween the two kinds of samples 4a and 4b.

The two kinds of samples 4a and 4b having respective values of fiberdiameter b different from each other as such were obtained by preparingtwo kinds of preforms, which used core members having an identicaldiameter, whose outside diameter ratios between core member and claddingmember differed from each other, and drawing them. Further, theperiphery of each of the two kinds of samples 4a and 4b is provided witha coating layer, made of the same material, having an outside diameterof 250 μm. The values of the mode field diameter MFD1, effective area,and chromatic dispersion are those measured at a wavelength of 1.55 μm.

For each of these two kinds of samples 4a and 4b, microbend loss wasmeasured by a method similar to that in the case of the firstembodiment. As a result, while the microbend loss of sample 4a having afiber diameter b of about 125μ was 4.12 dB/km, the microbend loss ofsample 4b having a fiber diameter b of about 150μ was 0.74 dB/km,whereby the latter was able to achieve the target value of microbendloss of about 1 dB/km or less, by which no increase in loss would begenerated by cabling.

FIFTH EMBODIMENT

FIG. 12 is a chart showing the refractive index profile in the fifthembodiment of the optical fiber according to the present invention. Theoptical fiber according to the fifth embodiment is adispersion-flattened fiber, in which the core region having an outsidediameter a is constituted by an inner core having a refractive index n1and an outside diameter of 3.75 μm, and an outside core provided on theouter periphery of the inner core and having a refractive index n2 (>n1)and an outside diameter of 8.25 μm. On the other hand, the claddingregion having an outside diameter b has a depressed cladding structureand is constituted by an inner cladding which is provided on the outerperiphery of the outer core and which has a refractive index n3 (=n1)and an outside diameter of 15.0 μm, and an outside cladding provided onthe outer periphery of the inner cladding and having a refractive indexn4 (>n3, <n2) and an outside diameter b.

The refractive index profile 600 shown in FIG. 12 indicates therefractive index of each part on the line L in FIG. 3A, in which parts610 and 620 show respective refractive indices in the core region 110and cladding region 120. Further, in the fifth embodiment, the relativerefractive index difference Δ⁺ of the outer core (refractive index n2)with respect to the outer cladding (refractive index n4) is +0.63%,whereas the relative refractive index difference Δ⁻ of each of the innercore (refractive index n1) and the inner cladding (refractive index n3)with respect to the outer cladding (refractive index n4) is −0.60%, eachgiven by the following expression (5):

Δ=(n_(core) ²−n_(cld) ²)/n_(cld) ²  (5)

In the above-mentioned expression (5), n_(core) is the refractive indexof the subject glass region, n_(cld) is the refractive index of theouter cladding taken as the reference. In expression (5), the refractiveindices of the individual glass regions can be placed in any order, sothat the relative refractive index difference of a region having arefractive index higher than that of the outer cladding becomes apositive value and is represented by Δ⁺, whereas the relative refractiveindex difference of a region having a refractive index lower than thatof the outer cladding becomes a negative value and is represented by Δ⁻.In this specification, the relative refractive index difference isexpressed in terms of percentage.

The samples prepared for evaluating the fifth embodiment consist of twokinds having fiber diameters of 125 μm and 160 μm, respectively. Also,the periphery of each of these samples is provided with a coating layer,made of the same material, having an outside diameter of 250 μm. Thoughtheir fiber diameters are different from each other, both of the sampleshave a chromatic dispersion value of 0.12 ps/nm/km at 1.55 μm, aneffective area of 72 μm² at a wavelength of 1.55 μm, and a cutoffwavelength of 1.187 μm. Also, their dispersion slope is 0.0096 ps/nm²/kmat a wavelength of 1530 nm, 0.0120 ps/nm²/km at a wavelength of 1550 nm,and 0.0265 ps/nm²/km at a wavelength of 1560 nm. Here, the dispersionslope refers to the gradient of the graph indicating the chromaticdispersion value in a predetermined wavelength band.

The microbend loss at a wavelength of 1.55 μm (1550 nm) was evaluated ineach sample and, as a result, was found to be 1.1 dB/km in the samplewith a fiber diameter of 125 μm and 0.1 dB/km in the sample with a fiberdiameter of 160 μm, whereby it was verified that the increase in losscaused by cabling was fully suppressed in the latter.

SIXTH EMBODIMENT

FIG. 13 is a chart showing the refractive index profile in the sixthembodiment of the optical fiber according to the present invention. Theoptical fiber according to the sixth embodiment is adispersion-compensating fiber in which the core region having an outsidediameter a is constituted by a single core having a refractive index n1and an outside diameter a1. On the other hand, the cladding regionhaving an outside diameter b has a depressed cladding structure and isconstituted by an inner cladding which is provided on the outerperiphery of the core and which has a refractive index n2 (<n1) and anoutside diameter b1, and an outer cladding provided on the outerperiphery of the inner cladding and having a refractive index n3 (>n2,<n1) and the outside diameter b.

The refractive index profile 700 shown in FIG. 13 indicates therefractive index of each part on the line L in FIG. 3A, in which parts710 and 720 show respective refractive indices in the core region 110and cladding region 120. Further, in the sixth embodiment, the relativerefractive index difference Δ⁺ of the core (refractive index n1) withrespect to the outer cladding (refractive index n3) and the relativerefractive index difference Δ⁻ of the inner cladding (refractive indexn2) with respect to the outer cladding (refractive index n3) are eachgiven by the above-mentioned expression (5).

The samples prepared for evaluating the sixth embodiment are those,among samples with Δ⁺=+0.9% and Δ⁻=−0.44% as shown in FIG. 14, havingcharacteristics indicated by point P in FIG. 14 in which the chromaticdispersion value at 1.55 μm (1550 nm) is −33 ps/nm/km, the dispersionslope at 1.55 μm (1550 nm) is −0.10 ps/nm²/km, the effective areaA_(eff) is 28 μm², and the ratio Ra (=a1/b1) of the outside diameter alof the core region to the outside diameter b1 of the inner cladding is0.6. They consist of two kinds having fiber diameters of 125 μm and 160μm, respectively. Also, the periphery of each of these samples isprovided with a coating layer, made of the same material, having anoutside diameter of 250 μm.

The microbend loss at a wavelength of 1.55 μm (1550 nm) was evaluated ineach sample and, as a result, was found to be 2.3 dB/km in the samplewith a fiber diameter of 125 μm and 0.3 dB/km in the sample with a fiberdiameter of 160 μm, whereby it was verified that the increase in losscaused by cabling was fully suppressed in the latter.

SEVENTH EMBODIMENT

Further, in the seventh embodiment, optical fibers having a furtherenlarged effective area were evaluated. While the prepared samples havea refractive index profile identical to that of FIG. 13, their effectivearea is 122 μm² (110 μm² or more). The prepared samples consist of twokinds having fiber diameters of 125 μm and 160 μm, respectively. Also,the periphery of each of these samples is provided with a coating layer,made of the same material, having an outside diameter of 250 μm. In bothof the samples, though their fiber diameters are different from eachother, the relative refractive index difference Δ⁺ of the core(refractive index n1) with respect to the outer cladding (refractiveindex n3) is +0.28%, the relative refractive index difference Δ⁻ of theinner cladding (refractive index n2) with respect to the outer cladding(refractive index n3) is −0.14%, the cutoff wavelength is 1.49 μm, theeffective area at a wavelength of 1.55 μm is 122 μm², the chromaticdispersion value at a wavelength of 1.55 μm is 22.1 ps/nm/km, and thedispersion slope at a wavelength of 1.55 μm is 0.062 ps/nm²/km.

The microbend loss at a wavelength of 1.55 μm (1550 nm) was evaluated ineach sample and, as a result, was found to be 1.3 dB/km in the samplewith a fiber diameter of 125 μm and 0.2 dB/km in the sample with a fiberdiameter of 160 μm, whereby it was verified that the increase in losscaused by cabling was fully suppressed in the latter.

As explained in the foregoing, since the optical fiber according to thepresent invention has a fiber diameter of 140 μm or more but 200 μm orless, the increase in microbend loss is effectively suppressed, and theprobability of breaking caused by bending stresses can be lowered tosuch an extent that it is practically unproblematic. Also, if theabsolute value of chromatic dispersion at a wavelength of 1.55 μm is 5ps/nm/km or less, and the Petermann-I mode field diameter is 11 μm ormore, then the occurrence of nonlinear optical phenomena is suppressed,whereby the optical fiber can suitably be used as an opticaltransmission line in WDM transmission systems utilizing the wavelengthband of 1.55 μm.

Without being restricted to the above-mentioned individual embodiments,the present invention can be modified in various manners. For example,the refractive index profile may have any structure without beinglimited to the double core structure and segmented core structure. Itcan also be realized by a ring core type refractive index profile inwhich a ring core region of a ring shape having a higher refractiveindex is provided around a center region having a lower refractiveindex.

In the present invention, as explained in the foregoing, since the fiberdiameter is 140 μm or more, the optical fiber has a high rigidity, sothat the increase in microbend loss is suppressed, whereas theprobability of the optical fiber breaking due to bending stresses ispractically unproblematic since the fiber diameter is 200 μm or less.Also, if the absolute value of chromatic dispersion at a wavelength of1.55 μm is 5 ps/nm/km or less, then the optical fiber is suitable forWDM transmissions in which this wavelength band is a wavelength in use.Further, in accordance with the present invention, since the mode fielddiameter is 11 μm or more, the optical energy density per unitcross-sectional area is so low that the occurrence of nonlinear opticalphenomena can be suppressed effectively. Therefore, the optical fiberaccording to the present invention is suitable as an opticaltransmission line in WDM optical transmission systems.

From the invention thus described, it will be obvious that theembodiments of the invention may be varied in many ways. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention, and all such modifications as would be obvious to one skilledin the art are intended for inclusion within the scope of the followingclaims.

What is claimed is:
 1. An optical fiber comprising a core regionextending along a predetermined axis and a cladding region provided onthe outer periphery of said core region, said core and cladding regionsbeing constituted by at least three layers of glass regions havingrespective refractive indices different from each other; said opticalfiber substantially insured its single mode with respect to light at awavelength in use; said optical fiber having, for at least onewavelength in the wavelength band in use, a chromatic dispersion of 5ps/nm/km or less in terms of absolute value and a Petermann-I mode fielddiameter of 11 μm or more; and said optical fiber having a fiberdiameter of 140 μm or more but 200 μm or less.
 2. An optical cablecomprising an optical fiber according to claim
 1. 3. An optical fiberhaving, for at least one wavelength in a wavelength band in use, adispersion slope of 0.02 ps/nm²/km or less in terms of absolute valueand an effective area of 50 μm² or more; said optical fiber having afiber diameter of 140 μm or more but 200 μm or less.
 4. An optical fiberaccording to claim 3, wherein said optical fiber has, at a wavelength of1550 nm, a chromatic dispersion of 5 ps/nm/km or less in terms ofabsolute value and a Petermann-I mode field diameter of 11 μm or more.5. An optical cable comprising an optical fiber according to claim
 3. 6.An optical fiber having, for at least one wavelength in a wavelengthband in use, a chromatic dispersion of −83 ps/nm/km or more but −18ps/nm/km or less and an effective area of 17 μm² or more; said opticalfiber having a fiber diameter of 140 μm or more but 200 μm or less. 7.An optical fiber according to claim 6, wherein said optical fibercomprises a core region extending along a predetermined axis and acladding region provided on the outer periphery of said core region,said core and cladding regions being constituted by at least threelayers of glass regions having respective refractive indices differentfrom each other, and wherein said optical fiber is substantially insuredits single mode with respect to light in the wavelength band in use. 8.An optical cable comprising an optical fiber according to claim
 6. 9. Anoptical fiber having, for at least one wavelength in a wavelength bandin use, a positive chromatic dispersion and an effective area of 110 μm²or more; said optical fiber having a fiber diameter of 140 μm or morebut 200 μm or less.
 10. An optical fiber according to claim 9, whereinsaid optical fiber has, at a wavelength of 1550 nm, achromaticdispersion of 5 ps/nm/km or less in terms of absolute value and aPetermann-I mode field diameter of 11 μm or more.
 11. An optical cablecomprising an optical fiber according to claim 9.